The present invention relates to photo cells and, more particularly, photo cells for use in photosensors for light level switching control.
Photo controllers are devices that automatically turn electrical devices on and off in response to the ambient light level. They are used, for example, on street lights to automatically turn them off during the day and on at night. They are also used on billboard lighting systems to turn the billboard lights on early at night, off late at night during periods of low vehicular traffic, on again during early morning rush hour periods when high traffic levels resume, and then off during the daylight hours. Photo controllers may also be used in reverse, for example, to turn a golf course water fountain on during the day and off at night.
Typical photo controllers use photosensors as a means to detect the ambient light level. Two common types of photosensors include either cadmium sulfide (CdS) photo cells or silicon junction devices (hereinafter “silicon sensors”).
Although the spectral response of CdS cells closely approximates the spectral response of a human eye, CdS cells tend to deteriorate rapidly in areas of high humidity, salt spray, or acidic air pollution again causing a drift toward longer burning hours caused by an earlier turn on and later turn off times. CdS cells also raise a potential disposal issue because of perceived cadmium hazards. Nonetheless, because of low initial cost and long history of use and human eye spectral response, CdS photo cells are still commonly used as a light sensor for photo controls.
An example of a conventional CdS photo cell and a photo control circuit using the same are shown in FIGS. 1A and 1B, respectively. As seen in FIG. 1A, the photo cell 100 includes a substrate (circuit board) 110 with a pair of leads 115 extending from a bottom surface 110a thereof. An upper surface 110b of the substrate 110 includes a CdS photosensitive region. As seen in FIG. 1B, the photo control circuit 150 includes a relay 155 having a heater resistor 157 coupled to the AC power neutral/white line through a CdS photo cell 160. The illustrated relay 157 is a normally closed relay.
Silicon sensors are also known. Such silicon sensors are also used in photo controls.